not much in my opinion. more important is that regardless of the grip your upper body is as stable as possible(knee on bench/arm on the top of an incline bench. I think with the "hammer" grip you can pull the bell higher but it's an individual thing. I do Barbell rows and one arm Dumbbell rows so i like the hammer grip to mix it up. i have a book where it shows that one arm dumbbell rows with a neutral grip gave more lat stimulation than seated cable rows. Note also the arm position in your pictures the upper picture the bell is close to the body and the lower pic the bells are wider from the body.

The one arm hammer DB row (I usually brace one hand on a bench, or whatever, rather than a knee on a bench) is a very underrated exercise, allowing a full stretch at the bottom. With the pull close to the body, it hit's the lat/middle portion of the back and lower traps very well, as noted by Donny. To a lesser degree, the rear delts.

With the wider DB row, knuckles facing forward,( can see the disadvantage of doing this with two arms rather than one arm, from the picture shown) the upper back, traps and rear delts very well. Not so much a direct influence on the lats themselves.

Most larger and stronger men will cheat greatly with one arm DB rows. And it pay's off for them extremely well. Cheating can be a productive tool, when preformed correctly. Like anything else, there is a right and wrong way of doing cheats. One of the more rewarding bicep movements are cheating curls, again when done correctly. Good Luck.

The one arm hammer DB row (I usually brace one hand on a bench, or whatever, rather than a knee on a bench) is a very underrated exercise, allowing a full stretch at the bottom. With the pull close to the body, it hit's the lat/middle portion of the back and lower traps very well, as noted by Donny.

Regarding "closer to the body" and hitting the middle portion of the back - what are your thoughts on an equally narrow barbell row in comparison?

i like to start wide and come in close as i do my sets...also try a supinated (underhand Grip). As i wrote a couple of times the barbell row is in my opinion one of the best moves you can do to build a thick back. I would even go as far to say some width too in the upper back. another thing in Barbell rows is using a thumbless grip and using your hands as "Hooks", advice often given in chins. I have never liked this myself and use a full grip as my little hands have trouble holding a heavy weight but try it and see if you get a diffrent feel. Hand and grip variations together with diffrent upper body angles can make Barbell rows very productive. Think i need some sledge training with Wooo, then i will have better grip strength...

i like to start wide and come in close as i do my sets...also try a supinated (underhand Grip). As i wrote a couple of times the barbell row is in my opinion one of the best moves you can do to build a thick back. I would even go as far to say some width too in the upper back. another thing in Barbell rows is using a thumbless grip and using your hands as "Hooks", advice often given in chins. I have never liked this myself and use a full grip as my little hands have trouble holding a heavy weight but try it and see if you get a diffrent feel. Hand and grip variations together with diffrent upper body angles can make Barbell rows very productive. Think i need some sledge training with Wooo, then i will have better grip strength...

BB Dazza: from personal experience, and watching and training with others, the close curl grip BB row is very productive. Probably the best version for most guy's (remembering also, that no one exercise will work equally well for everyone). Do what ever hand grip works for you. But the curl grip tends to hit most of the upper back/lats well. With the affect of adding mass and strength quite. Keep the elbows as close to the sides as possible, don't let them act as "wings".

With the DB row, close to the body, the hammer grip is used. Also an effective movement for back/lat development. Personal view (of whatever value); would always choose the BB version, of any rowing style, over the DB version. Only exception would be a DB row, where the elbows are way out and stay in line with the should girdle through the whole arc of the exercise. This will hit the rear delts strongly, as well as the traps. Just think of them as a rear DB raise, with a 90 degree bend at the elbow. And remembering to pull, or lead, with the elbow. And you get to use much more weight that a regular rear raise. This theory can be allied for lateral head delts also, with some other exercises..DB or BB.

BB'ers will use the hook/thumbless/false grip, quite a bit. BB'ing does not require a huge amount of weight in a rowing exercise, or pulldowns/chin's for that matter. The "hook" idea is to take the actually focus off the grip and on to the pull motion. Most BB'er will have too tight (and focuse) a thumb grip on most exercises, including benches/presses. Using a thumbless grip on benches is fairly standard for a lot of guy's. If wanting a thumb around the bar grip, than may be better served with a lighter, less tense grip. Good Luck.